Storage system using unformatted digital optical tape

ABSTRACT

Digital optical tape archival storage systems and methods are disclosed. A digital optical tape recorder simultaneously writes data and two or more guide tracks onto an unformatted digital optical tape recording medium. A digital optical tape reader includes a camera and an image processor. The camera captures a two-dimensional image of the digital optical tape recording medium including the data and the two or more guide tracks. The image processor extracts the data from the two-dimensional image based, at least in part, on the guide tracks.

RELATED APPLICATION INFORMATION

This patent is a continuation-in-part of U.S. patent application Ser.No. 14/919,530 filed Oct. 21, 2015, titled DIGITAL OPTICAL TAPE STORAGESYSTEM, which is a continuation-in-part of U.S. patent application Ser.No. 14/260,258 filed Apr. 23, 2014, titled DIGITAL OPTICAL TAPE STORAGESYSTEM, now U.S. Pat. No. 9,208,813, which claims priority fromprovisional patent application no. 61/815,650, filed Apr. 24, 2013,titled DIGITAL OPTICAL TAPE ARCHIVAL STORAGE SYSTEM, all of which areincorporated herein by reference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND

Field

This disclosure relates to data storage on digital optical tape.

Description of the Related Art

Color photographic recording film is unsuitable for long-term archivalstorages of motion pictures and other imagery. Color film containsphotographically active chemicals which remain in the film afterdevelopment. These chemicals will continue to undergo chemical reactionswhich cumulatively cause color fading in the images.

Conventional magnetic recording media are typically produced using acontinuous wet coating process, where a magnetic dispersion is appliedto the base film. This magnetic dispersion consists of binders, magneticpigments, dispersants and lubricants which are dissolved in organicsolvents to form a slurry. These binders (along with residual solvent,dispersants, etc.) will, over time, undergo chemical processes which mayweaken their ability to hold the magnetic pigments to the base media ormay cause breakdown of the magnetic pigments themselves due to chemicalreactions. Thus magnetic storage media may also be unsuitable forarchival storage of images and other data.

Digital optical tape system (DOTS) recording medium based on a verystable, very sensitive phase-change recording material has a potentialfor archival storage of imagery, data, and documents for periods inexcess of 100 years. It has been shown that DOTS recording medium isstable for 100 years at 38° C. and over 200 years at room temperature(25° C.). Further, DOTS recording medium is immune to chemical, water,and other environmental damage and is impervious to corrosion.

As described in U.S. Pat. No. 6,505,330, DOTS recording medium may beformed by coating an alloy of Antimony, Tin, and one or more additionalmetals onto a polymer film. The DOTS recording medium may be coated onthe film using a physical vapor deposition (PVD) process such as, forexample, DC magnetron sputtering. The film may be a dimensionally stablepolymer material such as, for example, polyethylene terephthalate (PET),commercially known as Mylar® or Estar®. The DOTS recording medium mayhave a thickness of about 0.08 μm and may be protected by an SiOxovercoat of about 0.095 μm thickness.

In contrast to conventional magnetic tape, a PVD coating technique doesnot require chemical binders or solvents and yields strong adhesionbetween the recording material and the base film. Thus DOTS recordingmedium is believed to be relatively immune to mechanical failures suchas delamination, chemical creep, fading, etc. suffered by conventionalmagnetic media over extended time periods.

Data may be written onto the DOTS recording medium by localized heatingusing a laser or other energy source. For example, U.S. Pat. No.7,248,278 describes a printing system that may be suitable for writingdata onto the DOTS recording medium. A linear spatial light modulator isilluminated by an expanded laser beam, and an image of the spatial lightmodulator is projected onto a photosensitive surface, which could be theDOTS recording medium.

Recorded data may be read from the DOTS recording medium by detectingthe localized optical reflectivity of the media. For example, U.S. Pat.No. 5,321,683 describes a system for reading the DOTS recording medium.A line of data on the DOTS recording medium is illuminated by a linearlyexpanded laser beam, and an image of the DOTS recording medium isprojected onto a detector array.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a digital optical tape recordingsystem.

FIG. 2 is a graphical representation of data recorded on DOTS recordingmedium.

FIG. 3 is a schematic block diagram of a digital optical tape readingsystem.

FIG. 4 is a schematic diagram illustrating the coverage of sensors usedto read a digital optical tape.

FIG. 5 is a flow chart of a process for recording and retrieving data onDOTS recording medium.

FIG. 6 is a graphical representation of estimating a position of afiducial marker.

Throughout this description, elements appearing in figures are assignedthree-digit reference designators, where the most significant digit isthe figure number where the element is introduced, and the two leastsignificant digits are specific to the element. An element that is notdescribed in conjunction with a figure may be presumed to have the samecharacteristics and function as a previously-described element havingthe same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

Referring now to FIG. 1, a digital optical tape recorder 100 may be usedto write data onto a DOTS recording medium 110. The DOTS recordingmedium 110 may be unformatted, which is to say the DOTS recording medium110 does not contain guide tracks, fiducial marks, or any other recordedfeatures prior to being recorded on by the digital optical tape recorder100.

The digital optical tape recorder 100 may include an energy source 140,beam forming optics 150 to direct light from the energy source 140 ontoa spatial light modulator 160, and imaging optics 170 to project animage of the spatial light modulator 160 onto the DOTS recording medium110. The DOTS recording medium 110 may be continuously or intermittentlyadvanced by a tape transport 120 driven by at least one motor 130. Thetape transport 120 is represented schematically in FIG. 1 as a capstan,but does not necessarily include a capstan. The tape transport 120 mayinclude a supply tape reel, a take-up tape reel, and other componentsnot shown in FIG. 1. The motor 130, the energy source 140, and thespatial light modulator 160 may be controlled by a write controller 180.

The energy source 140 may be a pulsed energy source providing periodicenergy pulse width suitable for writing on the DOTS recording medium110. For example, the energy pulse width may be 50 ns to 200 ns. Theenergy source 140 may be, for example, a laser having a wavelength fromless than 405 nm to greater than 830 nm. The energy source 140 may be aplurality of lasers or an array of lasers.

The spatial light modulator 160 may be a linear array of lightmodulating elements. The spatial light modulator 160 may be, forexample, a differential interferometric spatial light modulator asdescribed in U.S. Pat. No. 7,940,448. The spatial light modulator 160may be a diffractive spatial light modulator as described in U.S. Pat.No. 7,248,278. The spatial light modulator 160 may be a liquid crystalmodulator or some other form of spatial light modulator. In any case,the spatial light modulator 160 may be configured to modulate a lineararray of elements. The spatial light modulator may operate in a binarymanner such that each element in the linear array of elements may be setto be either “on” or “off”.

For example, the DOTS recording medium 110 may have a width of 12.7 mm(about one-half inch). Data bits may be recorded, for example, on therecording tape as 1 um diameter spots on 1.2 um centers. In thisexample, the spatial light modulator 160 may be a linear array of 10,000elements to record 10,000 spots across a central 12 mm portion of the12.7 mm tape width. The spatial light modulator 160 may have more orfewer than 10,000 elements. The recording tape width may be smaller orgreater than 12.7 mm. The number of elements in the spatial lightmodulator 160 may be substantially different from 10,000 elements forrecording tape widths other than 12.7 mm.

The beam forming optics 150 may accept energy emitted by the energysource 140 and form the energy into an elongate illumination spotdirected onto the linear array of elements of the spatial lightmodulator 160. The beam forming optics 150 may include an anamorphicbeam expander to create the elongate illumination spot. When the energysource 140 includes one or more lasers, the beam forming optics 150 mayinclude one or more apodizers, integrators, or other optical elements tosuppress the Gaussian beam profile emitted by the lasers and provideuniform power density across the illumination spot.

The imaging optics 170 may receive spatially modulated light from thespatial light modulator 160 and project an image of the spatial lightmodulator onto a surface of the DOTS recording medium 110. The width ofthe linear array of elements on the spatial light modulator 160 may begreater than or less than the width of the DOTS recording medium 110. Inthis case, the imaging optics 170 may provide magnification less than orgreater than one, such that the image formed on the DOTS recordingmedium 110 is smaller than the linear array of elements on the spatiallight modulator 160. The image formed on the recording tape may be alinear array of spots (more correctly potential spot locations), each ofwhich is controlled by a corresponding element on the spatial lightmodulator 160. Since the DOTS recording medium 110 is initiallyunformatted, a portion of the elements of the spatial light modulator160 may be dedicated to writing guide tracks and/or fiducial markersnecessary to read the data recorded on the tape. Each element of thespatial light modulator 160 may be used as a binary switch having twostates. In a first state, each element may direct light through theimaging optics 170 to write (i.e. to cause a phase change) thecorresponding spot on the DOTS recording medium 110. In a second state,each element may direct no light, or substantially less light, to thecorresponding spot on the DOTS recording medium 110 such that a phasechange does not occur. Where necessary, the imaging optics 170 mayinclude a polarizer, a beam splitter, an aperture, a spatial filter,and/or some other optical element to discriminate between the first andsecond states of the spatial light modulator.

A desirable feature of an archival data storage system is that the databe recoverable even if there is some physical degradation of therecording medium, such as warping or stretching of a tape recordingmedia. To enable data recovery from physically degraded recordingmedium, data may be written on the recording media in a format thatallows the data to be read without relying on precise mechanicalattributes of the recording media.

FIG. 2 shows an exemplary format for writing data on DOTS recordingmedium 210. Continuing the previous example, the width of the DOTSrecording medium 210 may be 12.7 mm. A central portion 220 of the widthof the DOTS recording medium 210 may be used for recording data 222 asan array of spots (not shown to scale) having either high or lowreflectivity. For example, the data 222 may be written as a grid of 1 umdiameter spots on 1.2 um centers. Each column of spots (as oriented inFIG. 3) in the central portion 220 may include, for example, 8192 spotsrepresenting 8192 binary bits of data.

Outside of the central portion 220 there may be two “dead bands” 236,246 that are not normally recorded. Outside of the dead bands and inset0.5 to 1 mm from the physical edge of the tape, may be two guide tracks230, 240. The guide tracks 230, 240 may be written onto the mediasimultaneously with the data 222. The guide tracks 230, 240 may includefiducial markers for reading the data 222. In this context, a “fiducialmarker” is an object written onto the digital optical tape recordingmedium for use as a reference when data recorded on the digital opticaltape recording medium is read.

For example, each guide track 230, 240 may include a continuous line ofspots extending along a length of the DOTS recording medium 210. Asshown at 242, the continuous line of spots may include a spot exactly inline with every column of data. As shown at 232, the continuous line ofspots may include a spot exactly in line with every other column ofdata. Additionally, each guide track 230, 240 may include additionalspots that form diagonal lines 234, 244 at periodic interval along thelength of the DOTS recording medium 210. Diagonal lines 244 may bewritten in an outer portion (i.e. between the continuous line of spots242 and the edge of the DOTS recording medium 210) of the guide track240 and diagonal lines 234 may be written in an inner portion (i.e.between the continuous line of sports 232 and the data 222) of the guidetrack 230. The asymmetry in the guide tracks 230, 240 may provide avisual reference of the “top” and “bottom” of the recording tape. Inaddition, the guide tracks 230, 240 may provide a visual reference ofthe intended direction of tape travel. For example, the diagonal lines234, 244, if extended as shown by the dashed lines 238, form series ofarrows pointing in the intended direction of tape travel.

As shown at 250, three columns of spots written across the entire widthof the DOTS recording medium 210 may be used as a beginning of tape(BOT) marker, and two sets of three columns opposing each other andwritten across the guide tracks 230, 240 and dead bands 2236, 246, butnot the central portion 220, may be used as a filemark to indicate bothbeginning of file (BOF) and end of file (EOF). The total number of spotswritten across the width of the DOTS recording medium can be determinedby counting the number of spots in each column of the BOT marker. Thenumber of data bits written across the central portion 220 of the widthof the DOTS recording medium 210 can be determined by subtracting thenumber of spots in each column of the BOF/EOF filemarks from the numberof spots in each column of the BOT marker.

The guide tracks 230, 240 are exemplary, and guide tracks for a digitaloptical recording medium may have different configurations of spots,lines, symbols, and other types of fiducial markers than those shown inFIG. 2. A digital optical recording medium may be written with two ormore guide tracks. For example, one or more additional guide tracks (notshown) may be written in the central portion 220 of the digital opticaltape recording medium. Additional guide tracks, when present, may besimilar to or different from the guide tracks 230, 240 of FIG. 2.

Referring again to FIG. 1, the write controller 180 may synchronize theoperation of the energy source 140 and the spatial light modulator 160.For example, the write controller 180 may transfer a line of data to thespatial light modulator 160 and then cause the energy source 140 to emita pulse of energy. In this case, transferring or writing data to thespatial light modulator 160 and pulsing the energy source 140 may occuralternately. The write controller 180 may also control the operation ofthe motor 130 and/or tape transport 120 such that the DOTS recordingmedium 110 moves before, after, or during the transfer of data to thespatial light modular and the DOTS recording medium 110 is stationarywhile the energy source 140 emits each pulse of energy. Alternatively,if the duration of each pulse of energy is short compared to the timerequired to move the DOTS recording medium 110 by the distance betweenadjacent columns of data, the DOTS recording medium 110 may movecontinuously.

The write controller 180 may include a data formatter 182 to convertdata to be recorded into the appropriate format for writing onto theDOTS recording medium 110. The data formatter 182 may encode the data tobe written, for example using an 8 B/10 B or 64 B/66 B encoding. Thedata formatter 182 may add error detecting and/or error correcting codesto the data to be written. The data formatter 182 may format the data insome other manner. The data formatter may divide the formatted data intolines of data bits to be written onto the DOTS recording medium.

The write controller 180 may include a guide track generator 184 togenerate the spots, lines, symbols, and other fiducial markers to bewritten onto the digital optical tape recording medium as guide tracks.The controller may include a data combiner 186 to combine the guidetracks and the formatted data such that each line of combined data 188sent to the spatial light modulator 160 includes a block of data to berecorded and the corresponding portions of the two or more guide tracks.

The write controller 180 may include digital logic circuits, memories,processors, and other circuits configured to perform the functionsdescribed herein. All or portions of the functions of the writecontroller 180 may be implemented in hardware which may include one ormore application specific integrated circuits and/or one or moreprogrammable gate arrays. All or portions of functions of the writecontroller 180 may be implemented by software executed by one or moreprocessors, such as a microprocessor, a graphics processor, or a digitalsignal processor.

Referring now to FIG. 3, a digital optical tape reader 300 may be usedto recover data from a DOTS recording medium 310. The DOTS recordingmedium 310 may store digital data and two or more guide tracks. Forexample, the DOTS recording medium 310 may be, or may be similar to, theDOTS recording medium 210 of FIG. 2. The digital optical tape reader 300may include an illumination source 340, a camera 350, and an imageprocessor 370 to extract data from the image captured by the camera 350.

The DOTS recording medium 310 may be continuously or intermittentlyadvanced by a tape transport 320 driven by at least one motor 330. Thetape transport 320 is represented schematically in FIG. 3 as a capstan,but does not necessarily include a capstan. The tape transport 320 mayinclude a supply tape reel, a take-up tape reel, and other componentsnot shown in FIG. 3.

The illumination source 340 may be any pulsed or continuous light sourcesuitable for reading the DOTS recording medium 310. The image sensordevices within the image sensor 360 may typically be fabricated fromsilicon, which exhibits peak photosensitivity in the near infraredportion of the electromagnetic spectrum and progressively lowerphotosensitivity at lower wavelengths in the visible spectrum. Theillumination source 340 may provide light within the visible or nearinfrared portions of the electromagnetic spectrum. The illuminationsource 340 may provide broadband (e.g. white) light or narrow bandlight. The use of narrow band light, for example from a light emittingdiode or laser, may simplify the design of the imaging optics 355 sincebroadband chromatic aberration correction may not be required with anarrow band light source. In general, the use of a lower wavelength mayalso simplify the design of the imaging optics 355 since thediffraction-limited spot size for a given lens numerical aperture isinherently smaller for a lower wavelength. However, silicon imagesensors are less sensitive at the lower end of the visible spectrum.Thus the resolution benefit of a lower wavelength may be balancedagainst a possible need for higher illumination power to offset lowerdetector sensitivity.

The illumination source 340 may include one or more lenses, mirrors, andother optical elements to concentrate the illumination of the area ofthe DOTS recording medium 310 to be read. The illumination source 340may include a polarizer or other optical elements to condition theillumination light to facilitate subsequent discrimination between lightreflected from written and unwritten spots on the optical recordingtape. The illumination source 340 may include one or more apodizers,integrators, or other optical elements to provide uniform illuminationpower density across the width of the DOTS recording medium 310.

The camera 350 may include imaging optics 355 and an image sensor 360.The imaging optics 355 may collect light from the illumination source340 that is reflected from the DOTS recording medium 310 and project animage of the surface of the DOTS recording medium 310 onto the imagesensor 360. When the image sensor 360 includes multiple image sensingdevices, the imaging optics 355 may contain separate imaging opticalelements for each image sensing device. The imaging optics 355 mayinclude one or more apertures, filters, polarizers, other elements todiscriminate between light reflected from written and unwritten spots onthe optical recording tape.

The image sensor 360 may be a charge coupled device (CCD) sensor or aphotodiode array sensor, of some other type of sensor. The image sensor360 may include a one-dimensional or linear array of photosensitivedetectors, a two dimensional array of photosensitive detectors, or maybe a combination of two or more arrays of photosensitive detectors. Theimage sensor 360 may include internal electronics to convert the lightlevel sensed by each detector into a serial signal or data stream.

The tape transport may continuously or incrementally move the DOTSrecording medium 310 past the camera 350 such that the camera 350captures a two-dimensional image of the surface of the DOTS recordingmedium 310. In this context, the term “capture” means to accumulate datarepresentative of the two-dimensional image. The captured image maycomprise an array of digital values, where each digital value representsthe reflectivity of a corresponding picture element or “pixel”. Thecaptured image may include areas of the DOTS recording medium 310 thatstore data and two or more guide tracks. The camera 350 may capture thetwo-dimensional image of the surface of the DOTS recording medium 310without requiring, or attempting, one-to-one alignment betweenindividual data bits recorded on the DOTS recording medium 310 andindividual photosensitive detectors within the image sensor 360.

The camera 350 may be configured to oversample the data recorded on theDOTS recording medium 310, which is to say the number of pixel valuescaptured for a given portion of the DOTS recording medium 310 may begreater than the number of data bits written within that portion of theDOTS recording medium 310. For example, the linear density of the pixels(i.e. the number of pixels per unit length or width of the recordingmedium) may be at least twice the linear density of recorded data bits.Continuing the previous example, the image sensor 360 may include alinear array of 20,000 detectors to read 8192 bits of data and theadjacent portions of the two or more guide tracks in each column on theDOTS recording medium 310. The image sensor 360 may include two or morecolumns of detectors which may be aligned or staggered. The image sensor360 may include two or more linear image sensor devices, each of whichreads a different portion of the recording tape.

The image sensor 360 may include one or more linear sensor device forsensing data recorded in a central portion of the DOTS recording medium310, and separate image sensor devices for sensing the two or more guidetracks. The image sensor devices for sensing the guide tracks mayinclude rectangular arrays of detectors. FIG. 4 shows the surface of anexemplary DOTS recording medium 410. A dashed rectangle 432 indicates anarea of the DOTS recording medium 410 sensed by a linear image sensor.Two dashed rectangles 432, 442 indicate areas of the DOTS recordingmedium 410 along the guide tracks 430, 440 sensed by corresponding guidetrack image sensors. In this example, the linear image sensor may haveabout 16500 detectors to sense 8192 dots or bits of information recordedin each column of data on the DOTS recording medium 410. The two guidetrack image sensors may have a rectangular array of, for example,512×512 detectors, 480×640 detectors, or some other number of detectors.

Referring back to FIG. 3, the image processor 370 may analyze a rollingwindow (i.e. a continuously or incrementally moving portion) of theimage captured by the camera 350 to extract the data stored on the DOTSrecording medium 310. The image processor 370 may be configured toextract the data based on the guide tracks. The image processor mayextract the data using the processes that will be describedsubsequently.

The image processor 370 may include digital logic circuits, memories,processors, and other circuits configured to perform the functionsdescribed herein. All or portions of the functions of the imageprocessor 370 may be implemented in hardware which may include one ormore application specific integrated circuits and/or one or moreprogrammable gate arrays. All or portions of functions of the imageprocessor 370 may be implemented by software executed by one or moreprocessors, such as microprocessors, graphics processors, and/or digitalsignal processors.

The function of the tape transport 330 and the camera 350 may besynchronized by a read controller 380. The linear density of capturedpixels along the length of the recording medium may depend on the speedat which the DOTS recording medium 310 moves past the camera 350. Theread controller 380 may control the operation of the motor 330 such thatthe DOTS recording medium 310 moves past the camera 350 at a rate thatallows the camera 350 to capture the desired number of pixels per unitlength. When the illumination source is a pulsed laser or other sourceconfigured to emit pulses of light, the read controller 380 may alsosynchronize the operation of the illumination source 340 and the camera350. The read controller 380 may include digital logic circuits,memories, processors, and other circuits configured to perform thefunctions described herein. All or portions of the functions of the readcontroller 380 may be implemented in hardware. All or portions offunctions of the read controller 380 may be implemented by softwareexecuted by one or more processors, such as a microprocessor or adigital signal processor. All or portions of the read controller 380 maybe implemented in one or more application specific integrated circuitsand/or one or more programmable gate arrays.

Description of Processes

Referring now to FIG. 5, a process for archival storage on digitaloptical tape (DOTS) recording medium may begin at 505 and end at 595.The actions from 510 through 540 cause data to written onto the DOTSrecording medium. The actions from 560 to 580 cause the recorded data tobe retrieved from the DOTS recording medium. The actions at 540 and 560may be separated by archival storage of the DOTS recording medium at 550for an indeterminate period of time. In some circumstances it may bedesirable to read the DOTS recording medium as it is being recorded tovalidate the recorded data. In this case the action at 560 may followthe action at 540 immediately without intervening storage.

Writing data onto the DOTS recording medium causes localizedirreversible (at least on a bit-by-bit basis) phase changes in therecording material. Thus the actions from 510 to 540 may only beperformed once on a given length of DOTS recording medium. The actionsfrom 510 to 540 may be performed, for example, by the digital opticaltape recorder 100 of FIG. 1.

At 510, data to be written onto DOTS recording medium may be formattedor converted into the appropriate format for writing onto the DOTSrecording medium. Formatting the data may include encoding the data, forexample using an 8 B/10 B or 64 B/66 B encoding. Formatting the data at510 may include adding error detecting and/or error correcting codes tothe data to be written. Formatting the data at 510 may includeprocessing the data in some other manner. Formatting the data at 510 mayinclude dividing the formatted data into lines of data to be writtenonto the DOTS recording medium.

At 520, guide tracks may be generated. Generating the guide tracks mayinclude generating spots, lines, symbols, and other fiducial markers tobe written onto the DOTS recording media for use as a reference whendata recorded on the DOTS recording medium is read. At 530, the guidetracks generated at 520 may be combined with the formatted data form 510such that each line of combined data to be written onto the DOTSrecording medium includes a block of formatted data adjacent portions oftwo or more guide tracks. The combined data may then be written onto theDOTS recording medium at 540 using an apparatus such as that shown inFIG. 1.

The actions from 510 to 540 may be performed sequentially, with eachaction completed before the subsequent action is initiated. Moreefficiently, the actions from 510 to 540 may be performed as acontinuous pipeline, such that some or all of the actions from 510 to540 proceed simultaneously, processing different lines of data to bewritten onto the DOTS recording medium.

Reading data from DOTS recording material does not alter the recordeddata or the material. Thus the DOTS recording material can be readrepeatedly, and the actions from 560 to 580 may be performed multipletimes for any given length of DOTS recording medium. The actions from560 to 580 may be performed, for example, by the digital optical tapereader 300 of FIG. 3.

At 560, a two-dimensional image of a surface of the DOTS recordingmedium may be captured by a camera. As described in conjunction withFIG. 3, the camera may include a lens that forms an image of the surfaceof the DOTS recording medium on a linear or two-dimensionalphotosensitive detector array. The DOTS recording medium may becontinuously or incrementally moved past the camera such that the cameracaptures the two dimensional image. The camera may be configured tooversample the data recorded on the DOTS recording medium, which is tosay a linear density of pixels captured by the camera across a width andalong a length of the DOTS medium may be greater than or equal to twotimes a linear density of data bits recorded on the DOTS medium. Thetwo-dimensional image may be captured at 560 by the camera withoutrequiring or attempting one-to-one alignment between individualdetectors within the camera and individual data bits stored on the DOTSoptical medium.

The two dimensional image captured by the camera at 560 may includerecorded data and two or more guide tracks, such as the guide tracks230, 240 shown in FIG. 2. At 570, an image processor may analyze thetwo-dimensional image captured by the camera at 560 to estimatepositions of fiducial markers within the two or more guide tracks. FIG.6 is graphical representation of estimating the location of a fiducialmarker. The left-hand graphic 610 is an enlarged portion of the guidetrack 240 which includes a continuous line of spot 612 along the lengthof a DOTS recording medium (the direction of travel of the DOTS mediumis vertical in FIG. 6) and a diagonal line of spots 614. The right-handgraphic is a representation of the corresponding portion of atwo-dimensional image captured by a camera. Since the camera is notprecisely aligned to the data spot positions on the DOTS recordingmedium, and since the camera oversamples the data, each spot of thecontinuous line of spots and the diagonal lines of spot is spread overseveral pixels in the two-dimensional image. However, line-detectionimage processing techniques can be applied to estimate positions 612,613 for the continuous lines of spots and the diagonal line of spots,respectively. Once the positions of both lines are estimated, theintersection of the two lines 614 may be determined to sub-pixelaccuracy. The intersection 614 may be used as a fiducial marker duringextraction of the data recorded on the DOTS medium.

Referring back to FIG. 5, at 580, data stored on the DOTS medium may beextracted based on the positions of fiducial markers estimated at 570.For example, a local coordinate system may be defined based on theestimated positions of the fiducial markers, and the data may beextracted based on the defined coordinate system.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of” respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. A digital optical tape archival storage systemcomprising: a digital optical tape recorder configured to simultaneouslywrite data and two or more guide tracks onto an unformatted digitaloptical tape recording medium; and a digital optical tape readercomprising: a camera to capture a two-dimensional image of the digitaloptical tape recording medium including the data and the guide tracks,and an image processor to extract the data from the two-dimensionalimage based, at least in part, on the guide tracks.
 2. The digitaloptical tape archival storage system of claim 1, wherein the camera isconfigured to oversample the data such that a linear density of pictureelements in the two-dimensional image captured by the camera is at leastdouble a linear density of bits in the data recorded on the digitaloptical tape recording medium.
 3. The digital optical tape archivalstorage system of claim 1, wherein the guide tracks include fiducialmarkers at periodic intervals along a length of the digital optical taperecording medium.
 4. The digital optical tape archival storage system ofclaim 3, wherein the image processor is configured to: estimate thepositions of the fiducial markers in the two dimensional image capturedby the camera, define a local coordinate system based on the estimatedpositions of the fiducial markers, and extract the data based on thedefined coordinate system.
 5. The digital optical tape archival storagesystem of claim 1, wherein the digital optical tape recorder furthercomprises: a spatial light modulator comprising a linear array ofindividually controllable elements to write a corresponding line of databits onto the digital optical tape recording medium; a guide trackgenerator to generate the two or more guide tracks; and a data combinerto combine data to be stored and the guide tracks such that each line ofdata bits written on the digital optical tape recording medium includesa block of data and a corresponding portion of each of the two or moreguide tracks.
 6. A method for archival data storage on digital opticaltape recording media, comprising: simultaneously writing data and two ormore guide tracks onto an unformatted digital optical tape recordingmedium; and capturing a two-dimensional image of the digital opticaltape recording medium with a camera, the two-dimensional image includingthe data and the two or more guide tracks, and processing thetwo-dimensional image to extract the data based, at least in part, onthe guide tracks.
 7. The method of claim 7, wherein the camera isconfigured to oversample the data such that a linear density of pictureelements in the two-dimensional image captured by the camera is at leastdouble a linear density of bits in the data recorded on the digitaloptical tape recording medium.
 8. The method of claim 7, wherein theguide tracks include fiducial markers at periodic intervals along alength of the digital optical tape recording medium.
 9. The method ofclaim 8, wherein processing the two-dimensional image to extract thedata further comprises: estimating the positions of the fiducial markersin the two dimensional image captured by the camera, defining a localcoordinate system based on the estimated positions of the fiducialmarkers, and extracting the data based on the defined coordinate system.10. The method of claim 1, wherein simultaneously writing data and twoor more guide tracks onto a digital optical tape recording mediumfurther comprises: modulating a light beam using a spatial lightmodulator having a linear array of individually controllable elements towrite a corresponding line of data bits onto the digital optical taperecording medium; generating the two or more guide tracks; and combiningdata to be stored and the guide tracks such that each line of data bitswritten on the digital optical tape recording medium includes a block ofdata and a corresponding portion of each of the two or more guidetracks.